This achievement can be unlocked pretty easily in the Fleeca Job Heist because it's a 2 player job and only a Gold and Platinum medal will be awarded, but the Platinum medal in this Heist will always be given to the Driver, no matter what happends. It is not influenced by headshots, accuracy or damage taken like the game tells you (this is only the case for this Heist though, almost all the other medals you can get during the next 4 Heist Finales and 19 Setups are based on player performance).
But generally the person who completed the objective (deliver this, do that, destroy this etc.) gets the Platinum medal.
To track your progress you want to go to Stats in the pause menu, Awards, and go over to the Heists tab. There should be an Award called "Decorated", that's the one that keeps track of your Platinum medals.
Anyway, back to the Fleeca Job Heist:
-During "Scope Out" (Setup) the Driver will always get the Platinum medal (the Driver is the person who gets in the driver seat first and drives around town).
-During "Kuruma" (Setup) the Driver will always get the Platinum medal (person who delivers the Kuruma to Lester).
-During the Heist Finale the Driver will once again always get the Platinum medal.
Of course you don't need to play the Fleeca Job Heist, you can unlock the achievement over time, but if you feel like grinding a bit this is the go to Heist!
If you completed the entire Fleeca Job Heist as a Leader you will have the ability to replay this Heist, just replay this Heist untill you earn 25 Platinum medals in total! If you don't have this Heist completed as a Leader you can join the Setups of other people or join your friends. But having that replay option as a Leader is really helpful!
You do have to wait a pretty long time before you can replay a Heist again. It is about 30 to 45 minutes. What you can also do if you don't like waiting so long is to cancel the Heist you are currently on and replay it instantly. You for example play the two Setup missions "Scope Out" and "Kuruma" and back out to Free Roam and call Lester to cancel the current Heist. After that you can call Lester again and the Replay Heist option will instantly be available! Playing the two Setup missions, cancelling the Heist and quickly replaying is faster compared to completing both the Setup missions and the Finale and waiting such a long time for the replay option to become available again. (Thanks to "TheBlackDragonX" for the awesome tip and "spacedvest2" for telling me to add this to the solution)
Note that Platinum medals are not Awards! You only need to get 25 medals (the ones that you earn at the end of a Setup or Heist). You do not have to get 25 Platinum Awards.